Schlossberg Scott, Chase Michael J, Griffin Curtice R
Elephants Without Borders, PO Box 682, Kasane, Botswana.
Department of Environmental Conservation, University of Massachusetts Amherst, Amherst, Massachusetts, United States of America.
PLoS One. 2016 Oct 18;11(10):e0164904. doi: 10.1371/journal.pone.0164904. eCollection 2016.
Accurate counts of animals are critical for prioritizing conservation efforts. Past research, however, suggests that observers on aerial surveys may fail to detect all individuals of the target species present in the survey area. Such errors could bias population estimates low and confound trend estimation. We used two approaches to assess the accuracy of aerial surveys for African savanna elephants (Loxodonta africana) in northern Botswana. First, we used double-observer sampling, in which two observers make observations on the same herds, to estimate detectability of elephants and determine what variables affect it. Second, we compared total counts, a complete survey of the entire study area, against sample counts, in which only a portion of the study area is sampled. Total counts are often considered a complete census, so comparing total counts against sample counts can help to determine if sample counts are underestimating elephant numbers. We estimated that observers detected only 76% ± SE of 2% of elephant herds and 87 ± 1% of individual elephants present in survey strips. Detectability increased strongly with elephant herd size. Out of the four observers used in total, one observer had a lower detection probability than the other three, and detectability was higher in the rear row of seats than the front. The habitat immediately adjacent to animals also affected detectability, with detection more likely in more open habitats. Total counts were not statistically distinguishable from sample counts. Because, however, the double-observer samples revealed that observers missed 13% of elephants, we conclude that total counts may be undercounting elephants as well. These results suggest that elephant population estimates from both sample and total counts are biased low. Because factors such as observer and habitat affected detectability of elephants, comparisons of elephant populations across time or space may be confounded. We encourage survey teams to incorporate detectability analysis in all aerial surveys for mammals.
准确统计动物数量对于确定保护工作的重点至关重要。然而，过去的研究表明，空中调查的观察员可能无法发现调查区域内目标物种的所有个体。此类误差可能会使种群估计值偏低，并混淆趋势估计。我们采用了两种方法来评估博茨瓦纳北部非洲草原象（Loxodonta africana）空中调查的准确性。首先，我们使用双观察员抽样法，即两名观察员对同一象群进行观察，以估计大象的可探测性，并确定哪些变量会对其产生影响。其次，我们将对整个研究区域进行的全面普查——总计数，与仅对部分研究区域进行抽样的样本计数进行了比较。总计数通常被视为完整的普查，因此将总计数与样本计数进行比较有助于确定样本计数是否低估了大象数量。我们估计，观察员仅探测到调查区域内2%的象群中的76%±2%的象群，以及87±1%的个体大象。可探测性随象群规模的增大而显著提高。在总共使用的四名观察员中，有一名观察员的探测概率低于其他三名，并且后排座位的可探测性高于前排。紧邻动物的栖息地也会影响可探测性，在更开阔的栖息地被探测到的可能性更大。总计数与样本计数在统计学上没有显著差异。然而，由于双观察员样本显示观察员遗漏了13%的大象，我们得出结论，总计数也可能低估了大象数量。这些结果表明，样本计数和总计数得出的大象种群估计值都偏低。由于观察员和栖息地等因素会影响大象的可探测性，不同时间或空间的大象种群比较可能会受到混淆。我们鼓励调查团队在所有哺乳动物的空中调查中纳入可探测性分析。
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